1. Field of the Invention
The present invention relates generally to a heating technique of hydrogen storage canister, and in particular to a method for heating up hydrogen storage canister to discharge hydrogen. The present invention also relates to a device for performing the method.
2. Description of the Prior Art
A fuel cell system is a power-generating unit that generates electrical power energy through electrochemical reaction of hydrogen and oxygen. To perform electrochemical reaction, hydrogen gas and air are separately conveyed to the fuel cell system via hydrogen gas passage and air passage.
Currently, a variety of ways are available for storage of hydrogen. Hydrogen can be stored in compressed gas form as compressing hydrogen, in liquid state as liquid hydrogen or in hydride form as metal hydride. Although a compressing hydrogen increases the gravimetric density of hydrogen, compressing hydrogen is an expensive process and the compressed hydrogen gas still occupies a great amount of space. In addition, the high pressure of compressed hydrogen is adverse to operation safety. Similarly, liquid hydrogen has high density. However, liquidization of hydrogen is a high energy consumption process. Moreover, because liquid hydrogen must be stored in a heat insulating storage tank, it is more economical and suitable to use liquid hydrogen in a system with a large storage tank. In most applications, hydride form is the most feasible way for storing hydrogen economically. An example is the compact hydrogen storage of electrical vehicle.
Metal hydride is formed by metallic material commonly referred to as hydrogen storage alloy which is capable to adsorb and desorb hydrogen. There are a variety of metal hydrides available. The charging/discharging pressure and temperature depend on the kind of metal hydride used. Also, the hydrogen storage capacity of metal hydride, i.e. the weight or volume of hydrogen that a unit weight of metal hydride can adsorb, varies from kind to kind. The conventional metal hydrides include lanthanum-nickel alloy (LaNi), iron-titanium alloy (FeTi) and magnesium (Mg) alloy, among which iron-titanium alloy is most commonly used. Some properties of iron-titanium alloy, including the hydrogen pressure, hydrogen flux and unit weight, make the iron-titanium alloy comparatively more suitable to be used in, for example, electrical vehicle than others.
The hydrogen storage capacity of magnesium alloy is larger than that of lanthanum-nickel alloy or iron-titanium alloy. In other words, for the same unit weight of alloy, magnesium alloy can store a larger amount of hydrogen than lanthanum-nickel alloy or iron-titanium alloy. However, magnesium alloy possesses a defect in practical use. Magnesium alloy is able to release a high flux of hydrogen only when the temperature is high enough e.g. at 200˜300° C. Therefore, it is not appropriate and inefficient to use magnesium alloy in a system that does not comprise a powerful heating device to heat up the magnesium alloy.
Generally, it is convenient and safe to use hydrogen storage alloy for storage of hydrogen in a fuel cell system. However, it should be noted that the ability of the hydrogen storage alloy to charge and discharge hydrogen directly affects the performance of the fuel cell system. Desorption of hydrogen is an endothermic process, and therefore, during discharging, the hydrogen storage alloy absorbs heat and causes a drop in temperature. The decrease of temperature in turn slows down the release of hydrogen from the hydrogen storage alloy. Therefore, to maintain a steady discharging rate or to speed up the discharging of hydrogen at high performance, it is required to properly heat up the hydrogen storage alloy.
The conventional techniques for heating up the hydrogen storage alloy to discharge hydrogen include heating by an electrical heater or by the heat waste recirculated from the engine or the fuel cell system. In that cases, the fuel cell system is either equipped with a power supply for powering the electrical heater, or alternatively, with a heat waste recirculator for utilizing of the heat waste. Both of the techniques are limited to use in some applications. Practically, the use of the electrical heater consumes a substantial amount of power and raises the operation cost. Besides, the heating rate is not fast enough. The heating devices currently available are not capable to heat up rapidly to a high temperature. On the other hand, the temperature of the heat waste from the proton exchange membrane fuel cell system is usually below 100° C. which is not hot enough for heating up the magnesium alloy based hydrogen storage alloy.